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Bureau of Mines information Circular/1986 



Reliability Prediction for Computerized 
Mine-Monitoring Systems 

By Raymond M. Kacmar and Edward F. Fries 




UNITED STATES DEPARTMENT OF THE INTERIOR 



\-- 



'^4U^ iixfer, i^mM'fli^^ 



Information Circular 9088 



Reliability Prediction for Computerized 
Mine-Monitoring Systems 

By Raymond M. Kacmar and Edward F. Fries 




UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Model, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



T 



3^5 







c\0 



%^ 



Library of Congress Cataloging in Publication Data: 



Kacmar, Raymond M 

Reliability prediction for computerized mine-monitoring systems. 

(Information circular; 9088) 

Bibliography: p. 13. 

Supt. of Docs, no.: I 28.27:9088. 



1. Mine safety-Data processing. 2. Mine gases-Data processing. 3. Mine fires-Data 
processing. 4. Reliability (Engineering). I. Fries, Edward F. II. Title. III. Series: Informa- 
tion circular (United States. Bureau of Mines); 9088. 



TN295.U4 



622 s [622'.8] 



86-600079 



CONTENTS 

Page 

Abstract ^ 1 

Introduction 2 

System descriptions 2 

Reliability predictions 2 

Methodology . 2 

Parts-count reliability 4 

Prior reliability data 4 

Reliability prediction assumptions 4 

Reliability models 5 

Prediction data and analysis 7 

Environmental impact on equipment 8 

Conclusion. . . « 8 

Bibliography 13 

ILLUSTRATIONS 

1. Block diagram of system A 3 

2. Block diagram of system B 3 

3. System A reliability configuration 5 

4. System B reliability configuration 6 

5. Sample form for assembly-rate summary 7 

6. Sample form for raicrocircuit failure-rate summary 8 

7. Gp environment for system A 9 

8. Gp environment for system B 9 

9. Data processing subsystem for systems A and B 10 

10. Communications subsystem for system A 10 

1 1 . Power subsystem for system A 10 

12. Transducer subsystem for system A 10 

13. Communications subsystem for system B 11 

14. Power subsystem for system B 11 

15. Transducer subsystem for system B 11 

16. System A reliability — adjusted 12 

17. System B reliability — adjusted 12 

TABLES 

1. Functional subsystems 6 

2. Environmental impact on equipment 11 



UNIT OF MEASURE ABBREVIATIONS USED IN THIS 


REPORT 


°C degree Celsius MM 


million 


h hour V 


volt 


Hz hertz 





RELIABILITY PREDICTION FOR COMPUTERIZED 
MINE-MONITORING SYSTEMS 



By Raymond M. Kacmar^ and Edward F. Fries^ 



ABSTRACT 

This report presents the Bureau of Mines research on the hardware 
reliability prediction for two Bureau monitoring projects. The basic 
concepts of reliability predictions are introduced along with the reli- 
ability models and assumptions used for this particular evaluation. The 
results of the reliability predictions are for ground-fixed and naval- 
sheltered environments. 



^tlectrical engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, PA 
(now with Defense Contracts Administrative Services, Los Angeles, CA) . 
'^Electrical engineer, Pittsburgh Research Center. 



INTRODUCTION 



By using computerized systems that are 
appearing in the marketplace, mine opera- 
tors now can monitor and display the sta- 
tus of underground conditions. However, 
before they buy, install, and depend on 
these systems they need to know how reli- 
able the data from the system will be; 
also, they need to predict how future ex- 
pansions will affect system reliability. 
The Bureau of Mines is conducting re- 
search to determine the reliability of 
current monitoring systems and also to 
demonstrate that standard reliability 
prediction techniques can be modified to 
predict the reliability of equipment op- 
erating in a mine environment. 

The mine monitoring systems researched 
for this report are electromechanical 



systems that remotely sense various envi- 
ronmental and operational parameters and 
transmit the data to a central location 
where the data are analyzed and/or dis- 
played. The equipment normally required 
to perform these functions consists of 
transducers, telemetry, one or more com- 
puters, and the associated peripherals 
and input-output interfaces. In the ba- 
sic system, the output from a transducer 
is converted to a format that enables a 
signal to be transmitted to a central 
computer station. There, a processor (or 
system of processors) tabulates the data, 
compares it with present alarm condi- 
tions, displays results, logs the data 
for future reference, and performs other 
calculations and data management. 



SYSTEM DESCRIPTIONS 



Both systems selected for these reli- 
ability predictions (to be referred to 
as system A and system B) have been in- 
stalled as research projects in conjunc- 
tion with the Bureau. System A is lo- 
cated in a research mine, owned by the 
Bureau, in Allegheny County, PA. System 
B is located in a commercial mine in In- 
diana County, PA. Although the system 
configurations are different, they are 
both representative of current tech- 
nology. The computer stations used for 
these systems were selected by the Bureau 
under competitive bid. They consist of a 
microprocessor-based controller with a 
cathode ray tube (CRT) and floppy disk 
drive and two printers. 

It should be noted that both systems 
are currently being modified to upgrade 
system capabilities. Therefore, the re- 
liability predictions in this report will 
only apply to the original system config- 
urations. However, these predictions can 
be easily adjusted once the hardware mod- 
ifications have been completed. 

The telemetry system used for sys- 
tem A employed a four-wire frequency 



shift-keyed communications link. Two of 
the wires were used to transmit data and 
two were used to receive data. On the 
surface, a telemetry card and local 
modem connected the computer to the 
telemetry line. At the sensing loca- 
tions, an analog-to-digital (A/D) con- 
verter and a remote modem connected the 
transducers. Previously, there were five 
communication "outstations" being used. 
One station was located on the surface 
and four stations were located under- 
ground. A block diagram of this system 
is shown in figure 1. 

For system B, two "trunk lines" (four- 
wire telemetry cables, each consisting of 
two power lines and two data lines) were 
used to monitor methane, carbon monoxide, 
and airflow. A block diagram of this 
system is shown in figure 2. 

The transducers sent measurements to 
the central station through an A/D tele- 
metry card. The analog voltage from the 
transducer is converted to an 8-bit digi- 
tal message and is then telemetered to 
the computer. 



RELIABILITY PREDICTIONS 



METHODOLOGY 



The standard procedures for performing 
reliability predictions are described in 



the military handbook (MIL-HDBK) 217D, 
"Reliability Prediction of Electronic 
Equipment." When possible, the methods 
in 217D, paragraph 5.1, are to be used. 

















Mine power 








Alarm 
PR 




Data 
PR 




1 on\/ 


60 Hz 




Battery 












Telemetry 

card 

and 

local 

modem 












1 




1 




1 






Surface 

outstation 

remote modem 

and A/D 






120 V 


UPS 




Computer 
console 


RS232 




60 Hz 


Interface 






























FSK line 


1 1 1 








CO 


CH4 


Air 








Surface 














Underground 
























































Mine 
outstation 

remote modem 
and A/D 




Mine 

outstation 

remote modem 

and A/D 




Mine 
outstation 

remote modem 
and A/D 




Mine 

outstation 

remote modem 

and A/D 




i 1 1 




1 1 1 




1 1 1 




1 1 1 




CO 


CH4 


Air 




CO 


CH4 


Air 




CO 


CH4 


Air 




CO 


CH4 


Air 



FIGURE 1.— Block diagram of system A. 





Alarm 
PR 




Data 
PR 


RS232 














1 


Power source 

and 

line driver 




Power source 

and 

line driver 


120V 


Computer 


60 Hz 


cor 


is 


ole 







Surface 



_ Lightning and_ 
surge protectors 

_Communication_ 
trunk lines 



Underground 

To other 

transducers 



JN 



CO 



JN 



JN 



CH4 



JN 



Air 



To other 
transducers 



JN 



JN 



CO 



JN 



CH4 



JN 



Air 



FIGURE 2.— Block diagram of system B. 



The equations contained in this section 
of the handbook require the detailed 
knowledge of the stress and quality lev- 
els of the components in the equipment 
under study. However, this detailed in- 
formation on the parts used for the moni- 
toring systems was not available for this 
analysis. Therefore, a combination of 
the following two methods was used to ar- 
rive at the predicted system reliability 
for the two configurations: parts-count 
and prior experience, 

Parts-Count Reliability 

When the methods of paragraph 5.1 can- 
not be used, MIL-HDBK 217D recommends 
the use of paragraph 5.2, "Parts-Count 
Reliability Prediction." This method is 
based on the type and quantity of the 
parts used. For this evaluation, it was 
used for equipment and assemblies whose 
parts lists were available or could be 
derived. This prediction procedure has 
limitations, and past experience has 
shown it to be conservative. Average 
stress levels are assumed for parts such 
that the system reliability can be ex- 
pressed as 

1 



MTBF (SYSTEM) = 



(1) 



I NiCXe.TTQ.) 
i=l 



Where for a given environment 

MTBF (SYSTEM) = the system average time 
between failure, h, 

Xq = generic failure rate for 
the i^*^ generic part, 
failures/MM h, 

Up = quality factor for the 
i^'^ generic part, 

Nj = quantity of the i^^ ge- 
neric part, 

and n = number of different ge- 
neric part categories. 

The following gives a sampling of the 
assumptions used for the parts-count 
prediction: 



1. Ambient temperature (T^) = 40° C 
for ground-fixed environment (Gp). 

2. Applied stress ratio = 0.5 = actual 
power dissipated per maximum rated 
power. 

3. Uses average complexities; e.g., 
any integrated circuit in 501-1,000 gate 
range uses 875 gates for calculation. 

4. A limited number of quality 
factors. 

Prior Reliability Data 

For this method, reliability data for 
similar parts used in other applications 
were used. This information was obtained 
from conversations with reliability ex- 
perts at the Department of Air Force, 
Rome Air Development Center (RADC), Reli- 
ability and Maintainability Engineering 
Section (RBER), Griff iss Air Force Base, 
New York. This method was used since 
several components and assemblies were 
not of the type that lend themselves 
to parts-count prediction techniques or 
could not be broken down into their con- 
stituent parts. As a result, the reli- 
ability of these assemblies were based on 
information obtained from RADC person- 
nel who have had experience with similar 
equipment in the past on Air Force sys- 
tems. Where complexities, environment, 
and/or duty cycles differed from that of 
the equipment under study, adjustments 
were made by RADC. 

RELIABILITY PREDICTION ASSUMPTIONS 

As a baseline for the prediction, other 
assumptions were necessitated as follows: 

1. Series model. — A series model was 
assumed such that a failure of any sub- 
system or assembly caused a system 
failure. 

2. Duty cycles. — All equipment was as- 
sumed to be operated at 100% of the sys- 
tem's operational time with the following 
exceptions: 

a. Alarm printer. — The alarm printer 
was assumed to operate only 10% of the 
time where the status printer was assumed 
to operate 100%. This assumption is 
based on the fact that the alarm printer 
only operates to output alarm conditions. 



b. Disk drive and controller. — These 
assemblies were assumed to be required 5% 
of the time because they are used only to 
Initialize the system. 

3. Environmental conditions. — The 
methodology for the prediction was in ac- 
cordance with MIL-HDBK 217D, which does 
not have a mine environment. The Envi- 
ronmental Impact section assesses the 
system reliability in different environ- 
ments. For the prediction baseline, a 
ground-fixed environment was assumed. 

4. Parts quality. — As a baseline, com- 
mercial plastic devices were assumed for 
integrated circuits and semiconductors 
(active devices). Passive devices such 
as resistors and capacitors were assumed 
to be of commercial quality. 

5. Sensing elements. — The reliability 
of sensing elements themselves was not 
considered. It was assumed that preven- 
tive maintenance made their failure rate 
contribution negligible. Electronics 



associated with the sensing function are 
included. 

6. Software reliability. — The reli- 
ability of the system software was not 
evaluated for this report. However, sim- 
ilar methods exist to determine software 
reliability but are beyond the scope of 
this report. 

RELIABILITY MODELS 

For this analysis, the two system con- 
figurations were separated into four 
functional subsystems: data processing, 
communications, power, and transducers. 
This functional designation was made to 
simplify future comparison between pre- 
dicted and actual operation. Table 1 
lists the assemblies composing the func- 
tional subsystems at each location. Fig- 
ures 3 and 4 indicate how the systems 
were configured for the reliability mod- 
els with each block (or assembly) being 



_ UPS 



JN 



Chassis 



CPU PWB 



Serial 
I/O PWB 



Disk 

and 

control 



CRT 

and 

control 



Power 
supply 



57o 



Console 



Printer 100% 



Printer 10% 



Tel em 
PWB 



5-V 
power 
supply 



24-V 
power 
supply 



Local 
modem 



JN 
and 
FSK 



15-V 
power 

supply 



5-V 
power 
supply 



Remote 
modem 



A/D PWB 



CO 



Airflow 



CH4 



Surface outstation 



Surface 



Mine 



r 



15-V 

power 

supply 



(4) 



5-V 

power 

supply 



Remote 
modem 



A/D PWB 



(4) 



CO 



(4) 




(4) 


Airflow 




CH4 



Mine outstation 



I 



FIGURE 3.— System A reliability configuration. 



required for the operational success of 
the system. Using this reliability mod- 
el and the four functional subsystems 



MTBF = 



previously discussed, the MTBF of 
system can be expressed as follows: 



1 



A(DPS) + X(COMM) + X(PS) + X(TRANS) 



the 



(2) 



where 



X(DPS) = failure rate for the 
data processing sub- 
system failures/MM h, 

X(COMM) = failure rate for the 
communication 
subsystem, 



and 



X(PS) 
X(TRANS) 



failure rate for the 
power subsystem, 

failure rate for the 
transducer subsystem. 



TABLE 1. - Functional subsystems 



Site subsystem 


System A 


System B 


Data processing 


1 console and 2 line printers. 


1 console and 2 line printers. 


Communications 


Telemetry with local modem: 


1 line driver assembly, 13 A/D 




1 surface outstation, 4 mine 


boards, 13 junction boxes. 




outstations, junction boxes, 


connectors. 




connectors. 




Power 


1 uninterruptible power sup- 


Transient protection, 2 junc- 




ply, junction boxes, power 


tion boxes, connectors. 




lines, connectors. 




Transducers .......... 


1 surface outstation and 
4 mine outstations with 


5 CO, 5 airflow, and 3 CH4 
transducers. 






1 each CO, airflow, and CH4 






transducers. 





1— — 




















1 

1 












Serial 
I/O PWB 




Disk 

and 

control 




CRT 

and 

control 


















Chassis 


— 


-CPU PWB 






Power 
supply 




Printer I0( 


1°^ 




- Printer 10% 














'— - 


















57o 


- — 


C 


on 


sole 


— 1 










r" 





- — 





-- 





— 1 


(2) 












Telenn 
PWB 


— 


24-V 
power 
supply 




Enclosure 


1 
1 


Transient 
protect 




JN 








1 

1 












p 


n\Mior cniirr 


e 


inW lino rlriuo 


1. 

r 1 








Surface 




Mine 




— 




(5) 




1 i 




(5) 





--. r- 


(3) 


1 








CO 


— 


A/D PWB 




1 

1 


Airflow 




A/D PWB 




CH4 - 


- A/D PWB 


1 
1 


JN 










1 
1 




1 
1 




















U _ 














_ J L. 














_J 







FIGURE 4.— System B reliability configuration. 



PREDICTION DATA AND ANALYSIS 

The data needed to perform the parts- 
count prediction were obtained from the 
schematics and parts lists of the various 
assemblies. A sample worksheet for the 
calculation is shown in figures 5 and 6. 
However, because of the proprietary na- 
ture of the monitoring systems, the ac- 
tual number and type of components used 
for the calculations are not shown. 

The failure rate ( ■ J for a partic- 
\ MTBF / 

ular group of similar components is then 
derived by multiplying the number of 
components by the generic failure rate 
(obtained from 217D) and the appropri- 
ate quality factor. As an example, the 



failure rate for 12 nonmilitary plastic 
resistors would be as follows: 

A = (NiXXeXTTQ), 

= (12) (0.0010(3), 

= 0.0396. 

The failure rate for a particular assem- 
bly is then obtained by adding the fail- 
ure rates for all of the components used. 
Likewise, the failure rate for a subsys- 
tem is obtained by adding the failure 
rates for each of the assemblies used. 
Finally, the predicted reliability of the 
system is derived using equation 2. 



Quality factors 



NAIIq 

Level 

1 



NAiiq 

Level 
2 



Fami 1y 



lype 

Transistor Si NPN 

Transistor SI PNP 

Diode SI gen purpose 

Diode zener 

LED 

Power transistor 

Composition RC 

Film RL 

Film RN 

Wi rewound RW 

Van WW RA 

Var comm RV 

Cerami c CX 

Tanti lum solid CSR 

Tantilum nonsolid CSR 

Al dry CE 

Low power pulse 

High power pulse/power 

Audio trans 

General purpose 

High current 

Circular/rack panel 

Printed wi re board 

2-sided board 

Toggle and push button 

Rotary 

Wi rewrap 

Wave solder 

Hand solder 

Crimp 



Qty. 



AG 



Jan 



Nonmi 1 
hermeti( 



Nonmi 1 
plastic 



NAIlQ 

Level 

3 



Semicondo. 



RES. 



CAP 

TFRS 

Relay 

Conn 

PCB 

Switch conn. 



Misc. 



.016 

.024 

.0031 

.012 

.033 
1.9 

.0011 

.016 

.018 

.18 
1.10 

.14 

.018 

.014 

.28 

.29 

.019 

.14 

.038 

.33 
1.1 

.017 

.024 

.0029 

.0029 

.96 

.000053 

.0006 

.0055 

.001 



1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 
1.0 



1.0 



5.0 
5.0 
5.0 
5.0 
5.0 
5.0 
3.0 
3.0 



3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
3.0 
6.0 
6.0 
3.0 
3.0 
1.0 
20.0 
50.0 



1.0 



10.0 

10.0 

10.0 

10.0 

10.0 

10.0 

3 

3 

3 

3 

3 

3.0 

3.0 

3.0 

3.0 

3.0 

3.0 

3.0 

3.0 

6.0 

6.0 

3.0 

3.0 

1.0 

20.0 

50.0 



1.0 



Microcircuits. 



From page 2 



8-2 



0-1/NH 



Drawing reference 



Assembly totals 



Assembly Name 



Site 



FIGURE 5.— Sample form for assembly-rate summary. 



Bits 




Qty. 


ROMS 


RAMS 




Hermetic 


Nonhermetic 


Qty. 


Hermetic 


Nonhermetic 




AGH 


NAGH 


AGNH 


nagnh 




AGH 


Nagh 


^iW^ 


\ GNH 


1.320 




.02 










.d3 




W 




321-576... 






.02 




.03 






.04 




.06 




577-1120.. 






.03 




.05 






.06 




.11 




1121-2240. 






.05 




.06 






.08 




.16 




2241-5000. 






.06 




.10 






.13 




.28 




5501-llK.. 






.09 




.18 






.29 




.76 




11001-17K. 






.12 




.23 






.44 




1.2 




17001-38K. 






.20 




.44 






.88 




2.5 




38001-74K. 






.33 




.84 






1.3 




3.9 










null 




mill 






lllllll 




llllllll 





Digital 



Linear 



Gates 


Qty. 


Hermetic 


Nonhermetic 




AGH 


Nagh 


AGNH 


Nagnh 


1.20 




.016 




.018 




21-50 




.020 




.023 




51-100 




.026 




.031 




101-500 




.044 




.06 




501-1000... 




.071 
.063 




.081 
.100 




1001-2000.. 




.11 




.19 




2001-3000.. 




.14 




.30 




3001-5000.. 




.20 




.50 




5001-7500.. 




.28 




.83 




7501-lOK... 




.43 




1.3 




10001-15K.. 




.61 




2.2 




15001-20K.. 




.85 




3.4 








mill 




mm 





Assembly Name 



Transistors 


Qty. 


Hermetic 


Nonhermetic 




>GH 


n>gh 


>GNH 


NifiNH 


1-32 

33-100 

101-300 




.031 
.084 
.29 




.049 

.16 

.79 








mm 




mm 





Summary 





Hermetic 


Nonhermetic 


Total 


total 


total 


Digital 






RAM 






ROM 






Linear 






Total IC's 






8-2x6.5 




//////////////// 


0x17.5 




//////////////// 


0-1x35 


////////////// 





Site 



FIGURE 6.— Sample form for microclrcult failure-rate summary. 



The results of the reliability predic- 
tions for the two configurations are 
shown in figures 7 and 8. The results 



on the assembly and subsystem levels are 
shovm in figures 9 through 15. 



ENVIRONMENTAL IMPACT ON EQUIPMENT 



The previous predictions for the sys- 
tems were performed using the MIL-HDBK- 
217D ground-fixed environment. However, 
as noted before, the handbook does not 
have a specific environmental factor that 
represents the operation of equipment in 
a mine atmosphere. It could be argued 
that this atmosphere is more severe than 
ground fixed because of the humidity 
and contamination-corrosion problems. On 
the other hand, it could be less severe 
owing to a lower ambient temperature. As 



a result of this uncertainty, the envi- 
ronmental impact on the prediction re- 
sults were reviewed. Table 2 shows how 
the MTBF varies as a function of environ- 
ment and the conditions that define the 
environment in MIL-HDBK 217D. Figures 16 
and 17 show the system reliability re- 
vised when a naval-sheltered environment 
is used for the equipment in the mine, 
and when a ground-fixed (Gp) environment 
is used for the surf ace 'equipment . 



CONCLUSION 



The predicted reliability of mine 
monitoring systems A and B using the 



methodology presented 
(on page 11) in hours; 



in this paper are 



UPS 


— 


JN 


?k = 50 


A=3 



Chassis 



CPUPWB 



Serial 
I/O PWB 



Disk 

and 

control 



CRT 

and 

control 



Power 
supply 



A =1807 



57o 



Console 



_ Printer 100% 



A = 400 



Printer 10% 



A = 40 



Tel em 
PWB 

I 

I A =200.1 



5-V 
power 
supply 



24-V 
power 
supply 



Local 
modem 



L 



JN 
and 
FSK 



A= I 



15-V 
power 
supply 



5-V 
power 
supply 



Remote 
modem 



A/D PWB 



CO 



A = 47.2 



Airflow 



A = 29.4 



CH4 



A =28.3 



191.5 



Surface outstation 



Surface 



Mine 



r 



(4) 



(4) 



Total failure | 
rate: 3894 | 

MTBF= 251 hi X = 191.5 



15-V 
power 
supply 



5-V 
power 
supply 



Remote 
modem 



A/D PWB 



CO 



\ = 47.4 



Mine outstation 



FIGURE 7.— Gp environment for system A. 



(4) 




(4) 


Airflow 




CH4 


X=29.4 


A =28.3 



Chassis 



A = 1807 



CPUPWB 



Serial 
I/O PWB 



Disk 

and 

control 



5% 



CRT 

and 

control 



Power 
supply 



Console 



Printer 100% 



A =400 



Printer 10% 



A=40 



(2) 



Telem 
PWB 



24-V 
power 
supply 






Enclosure 



Transient 
protect 



A= 1.84 



JN 



7^=0.495 



A = 37.49 



Power source end line driver 



Surface 



Mine 



(5) 



(5) 



(3) 



CO 



A = 31.59 



A/D PWB 



A = 23.18 



Airflow 



A=41 



A/D PWB 



A = 23.18 



CH4 



A =26.89 



A/D PWB 



A= 23.18 



JN 



A = 1.380 



I ! I I I ! 

Total failure rote = 3035.1 
MTBF= 329 h 



FIGURE 8.— Gp environment for system B. 



10 





■ 


" 


■ 




■ 


1 












Serial 
I/O PWB 




Disk 

and 

control 




CRT 

and 

control 
















Chassis 


— 


CPU PWB 






Power 
supply 




Printer 100% 




Printer 10% 




l^M.ll 


A = 1512.31 


A = 93.16 


>=25 


A = 90.41 


A=68.7I 


A =400 


)k = 40 




A_=I807 


4 










5% 




Console 











Total failure rate = A DPS " 2247 
MTBF = 445 h 

FIGURE 9.— Data processing subsystem for systems A and B. 



Telem 
PWB 



A= 13.07 



5-V 
power 
supply 



A=26.39 



24-V 
power 
supply 



X=63.I5 



Local 
modem 



A = 9753 



A =20014 



15-V 

power 
supply 



A = 6.29 



5-V 
power 
supply 



A = 26.39 



(3) 



Remote 
modem 



A= 8926 



A/D PWB 



A = 23.18 



_) L 



X= 191.48 



Surface outstction 



15-V 

power 

supply 



A=6.29 



5-V 
power 
supply 








(3) 




Remote 
modem 




A/D PWB 


A =26.39 


A = 89.26 


A = 23.I8 



n 



A= 191.48 



Mine outstation 



Total failure rate = AroMM "58.4 
MTBF= 863 h 

FIGURE 10.— Communications subsystem for system A. 



JN and 
conn 



A=I.O 



UPS 




Junctions 

and 
connectors 




A =50 


X = 3 



Total failure rate = ^pS = 53 
MTBF = 19,000 h 

FIGURE 11.— Power subsystem for system A. 





CO 




Airflow 




CH4 






>=47.4 


> = 29.4 


A = 28.3 












Surface 






Mine 




(4) 




(4) 




(4) 






CO 




Airflow 




CH4 






> = 47.4 


A = 29.4 


X = 28.3 





Total failure rate = ^ TRANS " 526.0 
MTBF= 1901 h 

FIGURE 12.— Transducer subsystem for system A. 



11 



TABLE 2. - Environmental impact on equipment 









Example 


MTBF 


MIL-HDBK 217 environment 


Environment definition 


Ta, °C 


part E 
(IC) 


adjust 
factor 


Ground-benign (Gb) 


Nearly zero environmental stress 
with optimum engineering oper- 
ation and maintenance. 


30 


0.38 


0.21 


Ground-fixed (Gp) 


Conditions less than ideal to 
include installation in per- 
manent racks with adequate 
cooling air, maintenance by 
military personnel, and possi- 
ble installation in unheated 
buildings. 


40 


2.50 


1.00 


Naval-sheltered (N s) 


Surface ship conditions similar 
to Gp but subject to occasional 
high shock and vibration. 


40 


4.00 


1.67 


Naval-unsheltered (Nj).... 


Nominal surface-ship-borne con- 
ditions but with repetitive 
high levels of shock and 


75 


5.70 


3.15 




vibration. 













(2) 


JN 




Telem 
PWB 




\= 1.38 


A= 12.18 




1 > 


= 37.49 



(2) 



24-V 
power 
supply I — 



X=6.4 



Enclosure 



A =0.33 



Power source and line driver 



(13) 



A/D PWB 



> = 23.18 



Total failure rate = ^cqivIM 
MTBF = 2939 h 



340.21 



FIGURE 13.— Communications subsystems for system B. 



(2) 



Transient 
protect 



^=1.84 



JN 



X = 0.495 



Total failure rate = "ApS - 4.175 
MTBF = 239,521 h 

FIGURE 14.— Power subsystem for system B. 



Environment 



Ground-fixed ^ . . . , 
Naval-sheltered . 



System A System B 



251 
209 



329 
283 



For equipment on surface. 
For equipment in mine. 

The actual reliability of monitoring sys- 
tems in the field will be compared with 
the predicted reliability once a suffi- 
cient data base has been established. 

Areas of potential improvements in 
reliability can be categorized as 



(5) 




(5) 




(3) 


CO 




Airflow 




CH4 






>= 31.59 

1 __j 


>=4I.I 


A = 26.89 



Total failure rate = ^JRANS ' 444.2 
MTBF = 2251.7 h 

FIGURE 15.— Transducer subsystem for system B. 



12 



UPS 



7^ = 50 



JN 



A = 3 



Chassis 



CPU PWB 



Serial 
I/O PWB 



Disk 

and 

control 



CRT 

and 

control 



Power 
supply 



> = 1 807 



Console 



Printer I007o 



> = 400 



Printer I07o 



A = 40 



Tel em 
PWB 



5-V 
power 

supply 



24-V 
power 
supply 



Local 
modem 



X =200.1 



JN 
and 
FSK 



■K= I 



15-V 
power 
supply 



5-V 
power 
supply 



Remote 
modem 



A/D PWB 



CO 



I = 47.4 



Airflow 




CH4 


A = 29.4 


A = 28.3 



I = 191.5 



Surface outstation 



Surface 



Mine 



r 



Total failure 
rate: 4780 
MTBF= 209 hi > = 319.8 



15-V 
power 

supply 



(4) 



(4) 



5-V 
power 
supply 



Remote 
modem 



A/D PWB 



CO 



A = 79.2 



14) 




14; 


Airflow 




CH4 


A=49.2 


A =47.3 



Mine outstation 



FIGURE 16.— System A reliability— adjusted. 



Chassis 



A = I807 



CPU PWB 



Serial 
I/O PWB 



Disk 

and 

control 



5% 



CRT 

and 

control 



Power 
supply 



Console 



Printer 100% 



A =400 



Printer 10% 



A = 40 



Telem 
PWB 



24-V 
power 
supply 



Enclosure 



A =37.49 



Power source and line driver 



(2) 



Transient 
protect 



1= 1.84 



JN 



A =0.495 



(each) 



Surface 



Mine 



(5) 



(5) 



(3) 



CO 



A = 52.75 



A/D PWB 



A =38.7 



Airflow 



A = 68.64 



A/D PWB 



A = 38.7 



CH4 



A = 44.91 



A/D PWB 



A= 38.7 



I. 



Total failure rate = 3535.8 
MTBF = 283 h 



JN 



A=2.3I 



FIGURE 17.— System B reliability— adjusted. 



part-quality upgrade and elimination of 
reliability design deficiencies. As pre- 
viously defined in the prediction as- 
sumption, the baseline prediction was 
performed using standard-quality parts. 
Sensitivity to part-quality as a poten- 
tial means of reliability is a proven 
fact based on field experience. Reli- 
ability of the system can be improved by 
doing the following: 

Use hermetic devices in an atmos- 
phere as corrosive and humid as that of a 
mine. 

Use MIL-grade connectors and solder 
connections instead of integrated-circuit 
sockets. 



13 



o Establish a well-defined inspection 
and preventive maintenance schedule. 

All equipment should be burned-in at 
an elevated temperature for at least 
100 h with the last 24 h failure free. 

All printed circuit boards exposed 
to the mine environment be conformally 
coated to prevent moisture and corrosion 
problems . 

System developments should involve 
front-end design consistent with the cri- 
ticality of operation and safety in a se- 
vere environment. 



BIBLIOGRAPHY 



Donald, L. B., and R. M. Baker. An 
Annotated Bibliography of Coal Mine Fire 
Reports (contract J0275008, Allen Corp. 
of Am.). V. 1, BuMines OFR 7(l)-80, 
1979, 98 pp., NTIS PB 80-140205; v. 2, 
BuMines OFR 7(2)-80, 1979, 400 pp., NTIS 
PB 80-140213; v. 3, BuMines OFR 7(3)-80, 
1979, 654 pp., NTIS PB 80-140221. 

Henley, E. J., and H. Kumamoto. Proba- 
bilistic Parameters of Whole Processes. 
Sec. 4.2.5 in Reliability Engineering and 
Risk Assessment. Prentice-Hall, 1981, 
p. 180. 

Kacmar, R. M. Reliability of Comput- 
erized Mine-Monitoring Systems. BuMines 
IC 8882, 1982, 10 pp. 

U.S. Air Force, Reliability Analysis 
Center. Rome Air Development Center Re- 
liability Design Handbook. RDH375, Mar. 
1976, pp. 19-21, 292. 

U.S. Army Aviation Systems Command. 
Introduction to Reliability. Ch. 1.0 in 
Pocket Handbook on Reliability. Sept. 
1975, p. 11. 



U.S. Code of Federal Regulations. Ti- 
tle 30 — Mineral Resources; Chapter I — 
Mine Safety and Health Administration, 
Department of Labor; Subchapter — Coal 
Mine Safety and Health; Part 75 — Manda- 
atory Safety Standards — Underground Coal 
Mines; July 1, 1983. 

U.S. Code of Federal Regulations. Ti- 
tle 30 — Mineral Resources; Chapter I — 
Mine Safety and Health Administration, 
Department of Labor; Subchapter D — Elec- 
trical Equipment, Lamps, Methane, De- 
tectors; Tests for Permissibility; Fees; 
Part 18 — Electric Motor-Driven Mine 
Equipment and Accessories; July 1, 1983. 

U.S. Mine Safety and Health Administra- 
tion (Dep. Labor). Injury Experience in 
Coal Mining. Annu. Inf. Reps. 1973-80. 

Watson, R. A. An Intrinsically Safe 
Environmental Monitoring System for Coal 
Mines. Proc, 6th Conf . on Coal Mine 
Electrotechnol. , WV Univ., Morgantown, 
WV, July 1982, pp. 345-360. 



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